It is well known that freedom from distraction is an important consideration for workers satisfaction with their office environment. In a conventional enclosed office with full height partitions and doors, any speech sound intruding from outside the office is attenuated or inhibited by the noise reduction (NR) qualities of the wall and ceiling construction. Residual speech sound actually entering the office is normally masked or covered up by even very low levels of background noise, such as from the building heating or ventilating system. Under normal circumstances, the resulting speech audibility is sufficiently low that the office worker is unable to understand more than an occasional word or sentence from outside, and is therefore not distracted by the presence of colleagues' speech. In fact, it was shown more than 35 years ago that a standardized objective measure of speech intelligibility called the articulation index, or AI, could be used to reliably predict most people's satisfaction with their freedom from distraction in the office. "Perfect" intelligibility corresponds to an AI of 1.0, while "perfect" privacy corresponds to an AI of 0.0. Generally, office workers are satisfied with their privacy conditions if the AI of intruding speech is 0.20 or less, a range referred to as "normal privacy".
In recent years, the open plan type of office design has become increasingly popular due to its obvious flexibility and communication advantages. In contrast to conventional closed offices, the open plan design has only partial height partitions and open doorways, and unwanted speech readily transmits from a talker to unintended listeners in adjacent offices. Limited acoustical measures can be employed to reduce the level of the resulting speech that is transmitted. Highly sound absorptive ceilings reflect less speech, and higher partitions diffract less sound energy over their tops. Additionally, doorways are placed so that no direct line of sight or sound transmission exists from office to office, and the interiors of offices are treated with sound absorptive panels. Nevertheless, even in an acoustically well designed open office, the sound level of intruding speech is substantially greater than in most enclosed offices. In order to obtain the normal privacy goal of 0.20 AI, acousticians know that the level of background sound in the open office must be raised, usually by electronic sound masking systems. Indeed, a considerable proportion of larger contemporary open offices use electronic sound masking systems, sometimes called "white sound" systems. However, few smaller offices use such systems due to prohibitive costs.
Conventional sound masking systems typically comprise four main components; an electronic random noise generator, an equalizer or spectrum shaper, a power amplifier, and a network of loudspeakers distributed throughout the office. The equalizer adjusts the spectrum to compensate for the frequency dependent acoustical filtering characteristics of the ceiling and plenum or air space above and to obtain the spectrum shape desired by the designer. The power amplifier raises the signal voltage to permit distribution to the loudspeakers without unacceptable loss in the network lines. The generator, equalizer, and power amplifier are typically located at a central location connected to the loudspeaker distribution network. A typical system uses loudspeakers serving about 100-200 square feet each (i.e. placed on 10' to 14' centers); the loudspeakers are usually concealed above an acoustical tile ceiling in the plenum space. In most cases, the plenum above the ceiling is an air-return plenum so that the loudspeaker network cable must be enclosed in metal conduit or use special plenum-rated cable in order to meet fire code requirements.
The goal of any sound masking system is to mask the intruding speech with a bland, characterless but continuous type of sound that does not call attention to itself. The ideal masking sound fades into the background, transmitting no obvious information. The quality of the masking sound is subjectively similar to the natural random air turbulence noise generated by air movement in a well-designed heating and ventilating system. The overall shape of the masking spectrum is of paramount importance if the goal of unobtrusiveness is to be met. If it has any readily identifiable or unnatural characteristics such as "rumble," "hiss," or tones, or if it exhibits obvious temporal variations of any type, it readily becomes a source of annoyance itself. However, if the sound has a sufficiently neutral, unobtrusive spectrum of the right shape, it can be raised, without becoming objectionable, to a sound level or volume nearly equal to that of the intruding speech itself, effectively masking it.
Although a distributed, ceiling mounted sound masking system has numerous advantages, such a system has significant disadvantages that interfere with the effectiveness of the system at the level of the individual office worker. For example, mechanical system ducts and other physical obstructions, as well as acoustical variations in the above-ceiling plenum and ceiling components such as vented light fixtures and air return grilles, pose significant challenges to the designer in achieving adequately uniform spectral quality. In many installations, cavity resonances in the plenum occur and cannot be completely ameliorated by equalization or other techniques. As a consequence, the acoustical spectrum obtainable at any one office worker location may be substantially compromised compared to the ideal spectrum desirable at his or her particular location. This non-ideal spectrum and spatial variation throughout the office places an effective upper limit on the effectiveness of the masking system.
Obtaining the correct level or volume of the masking sound also is critical. The volume of sound needed may be relatively low if the intervening office construction, such as airtight full height walls, provides high NR, but it must be relatively high in level if the construction NR is compromised by partial-height intervening partitions or acoustically poor design or materials. Even in an acoustically reasonably well designed open office, the level of masking noise necessary to meet privacy goals may be judged uncomfortable by some individuals, especially those with certain hearing impairments. Some systems use volume controls on each masking loudspeaker to permit their adjustment for good spatial uniformity. Even with this costly measure, variations in level of 3-6 dB throughout an office are typical. This amount of variation typically corresponds to differences in AI of 0.1 to 0.2 and sentence intelligibility differences of more than 80% at different locations throughout the office. Such variations are clearly undesirable. Additionally, masking noise may not be desired in larger conference rooms or other communication spaces sharing ceiling plenums with masked areas, and it is impossible for the designer to fully satisfy both requirements.
Subjective spatial quality is a third important attribute of sound masking systems. The masking sound, like most other natural sources of random noise, must be subjectively diffuse in quality in order to be judged unobtrusive. Naturally generated air noise from an HVAC system typically is radiated by many spatially separated turbulent eddies generated at the system terminal devices or diffusers. This spatial distribution imparts a desirable diffuse and natural quality to the sound. In contrast, even if a masking system provides an ideal spectrum shape and sound level, its quality will be unpleasantly "canned" or colored subjectively if it is radiated from a single loudspeaker or location. A multiplicity of spatially separated loudspeakers radiating the sound in a reverberant (sound reflective) plenum normally is essential in order to provide this diffuse quality of sound. With some non-reflective ceiling materials and fireproofing materials used in plenums, it is necessary to resort to two or more channels radiating different (incoherent) sound from adjacent loudspeakers in order to obtain a limited degree of diffuseness. Some contemporary masking systems use such techniques, adding significantly to their installation complexity and cost. Despite careful consideration and design, the degree of diffuseness typically obtained is further limited by the economically dictated need to place many of the ceiling loudspeakers on the same signal distribution channel.
Finally, intentional lack of any user accessible controls is a requirement of conventional masking system design. Because the background sound affects the privacy of all occupants in the office, it is not appropriate to permit individual users to control the characteristics of the masking sound, which are relatively critical. Any temporal changes in the masking level throughout the office are seriously objectionable. Controls are typically locked by various security devices, including physical cabinet locks and electronic password controls to generators and other centrally located electronic components.
In addition to the conventional sound masking systems described above, several self-contained general-purpose devices have been used to provide masking sound in offices. These include mechanical devices using fans and various types of electronic sleep aids and "ambient nature environment" units. Although some of these devices have incorporated "white noise" generators, no one system is able to provide the three essential characteristics, for sound masking application, of tailored spectral shaping, adjustable level, and diffuse spatial quality.